1. Field of the Invention
This invention relates to check valves providing metal-to-metal sealing, and more particularly to a metal seal or sealing structure for a check valve effective for sealing at both low fluid pressures and at high fluid pressures over a wide differential pressure range.
2. Description of Prior Art
Heretofore, check valves have been provided with metal seals or metal sealing arrangements. While many of the prior art metal sealing structures are effective for sealing at either high or low fluid pressures, it has been found that most of such metal sealing structures are not very effective at both high and low fluid pressures.
Typical prior art metal sealing arrangements for check valves are illustrated in FIGS. 1A, 1B and 1C. FIG. 1A illustrates the "differential angle" sealing principle. In this design, the sealing faces theoretically engage one another as a continuous point contact around the circumference of a circle rather than as a conical face as would be the case if the mating face angles matched. Low pressure sealing is achieved since appropriately high contact stresses can be generated even by a relatively small force, such as can be generated by a mechanical spring and/or low pressure. The drawback of the design shown in FIG. 1A is that at higher pressure, the force generated on the contact faces is great enough to permanently yield/deform them. An effective "fit" is created between the mating parts as a result of this process (typically referred to as "coining"), and a very good "high" pressure seal is also achieved, but the contact area is also increased. With the bearing area thus increased, the amount of force required to generate sufficient contact stress to ensure a seal is also increased, typically beyond the capacity of the mechanical device (typically a spring), provided to generate the low pressure seal. This design is simple and cost effective to manufacture and can be used for low or high pressures (but not both in the same application). Specifically, repeatable low pressure sealing is typically lost once high pressure is applied.
Oil field applications for check valves vary widely. The application which tends to create the most problems seems to be related to Christmas trees where the check valves are often used as one of the two "fail close" barriers required by code for isolation of chemical injection ports. In this application, the check valves must seal repeatedly any differential pressure above ambient and up to the rated maximum working pressure of the system to which they are attached. This is a range of up to 15,000 psi or more.
FIGS. 1B and 1C illustrate two configurations of a principle which might be referred to as the "soft seat/hard seat" sealing principle. Both concepts incorporate a relatively soft, easily deformable element to provide the low pressure seal and a shouldering interface providing a higher pressure metal seal. As pressure is increased across the poppet or check valve member in these designs (in their intended sealing direction), the soft seat sealing elements are progressively deformed until the poppet contacts the body seat. The soft seat most likely continues to provide sealing integrity throughout the operational pressure range, although it has been demonstrated for most designs that a true metal-to-metal seal is also achieved at higher pressures (i.e., once metal contact stresses have reached the minimum critical level to initiate a seal). The soft seats shown in FIGS. 1B and 1C are "resilient" in the sense that so long as they are not deformed beyond certain material-specific limits (as ensured by the shouldering of the metal contact faces) they will return substantially to its pre-deformed geometry. This gives these check valves the ability to provide repeatable low pressure sealing. The metal "shoulder" provides repeatable higher pressure sealing (although the metal sealing "range" is limited as is the case for the "differential angle" design shown in FIG. 1A). A disadvantage of the combined soft seat/hard seat designs is the necessity of a non-metallic low pressure sealing element, which is undesirable for some end users of check valves, particularly where high temperatures are involved or where the valves are exposed to harmful chemicals.
It is an object of the present invention to provide a metal sealing arrangement for a check valve which provides staged sealing for operation over a wide differential pressure range with a relatively long life.